In an optical communication field and an optical measurement field, an optical modulation module such as an optical modulator in which an optical waveguide and a control electrode for controlling light waves propagating through the optical waveguide are provided in a substrate is frequently used.
Such an optical modulation module has, for example, a configuration in which two sets of Mach-Zehnder type optical waveguides are provided on a substrate having an electro-optic effect, a plane of polarization of one or both of light waves (linearly polarized lights) emitted from the respective optical waveguides is rotated, polarization-combining is performed in the relationship that the planes of polarization are orthogonal to each other, and the polarization-combined light is output.
With regard to the polarization-combining as described above, Patent Literature No. 1 discloses a configuration in which rutile is used for the polarization-combining. However, the full length of a polarization-combining optical system becomes longer, and therefore, there is a problem in which a reduction of the size of an optical modulation module is difficult.
In contrast, as in Patent Literature No. 2 or Patent Literature No. 3, by using a polarizing beam splitter (PBS) for the polarization-combining, it becomes possible to reduce the size of an optical modulation module, compared to a case of using rutile.
However, in Patent Literature No. 2, a configuration is made in which two input surfaces of making light waves be input to the PBS are in the positional relationship of being orthogonal to each other and the light wave on one side is reflected by a reflecting mirror which is a separate body from the PBS, and then led to the PBS. Therefore, there is a problem of lack of the stability of an optical axis. Further, in Patent Literature No. 3, although a PBS with a reflecting mirror integrated therewith is disclosed, how to fix a wavelength plate is not fully considered.
In FIG. 1, similar to Patent Literature No. 3, a polarization-combining optical system configured by using a wavelength plate and a PBS with a reflecting mirror integrated therewith is shown. In the drawing, a ½ wavelength plate 3 is provided on one of two optical paths in which two light waves (linearly polarized lights) which are emitted from an emitting lens array 2 connected to emitting ends of two optical waveguides 1 reach a PBS 4.
Due to this configuration, in a case where two polarized lights (hereinafter referred to as horizontally polarized lights) L1 and L2 each having a polarization direction parallel to the plane of the paper are emitted from the emitting lens array 2, the horizontally polarized light L1 on one side is polarization rotated by 90 degrees at the λ/2 wavelength plate 3, thereby being converted into a polarized light having a polarization direction perpendicular to the plane of the paper (hereinafter, referred to as a vertically polarized light L3), and is input to the PBS 4. The horizontally polarized light L2 on the other side is directly input to the PBS 4 and is combined by the PBS 4. A combined light L4 which is obtained by such polarization-combining is output through an emitting collimator 7 composed of a condensing lens 5, a ferrule 6 with a polarization maintaining fiber, and the like.
However, in a case of using a very thin wavelength plate having a thickness of the order of several tens of μm, it is difficult to fix the wavelength plate at a desired angle, and it is difficult to maintain the parallelism between the wavelength plate and the PBS. For this reason, depending on the slope of the wavelength plate, the light wave after transmission through the wavelength plate does not become a complete linear polarization, and thus there is a problem in which an optical loss occurs in the PBS, or the light wave is shifted, whereby a coupling loss occurs in the emitting collimator 7.
Further, it is also conceivable to integrate the wavelength plate and the PBS by bonding the wavelength plate to the PBS. However, the wavelength plate is very thin and thus handling thereof is difficult, and it is difficult to maintain the parallelism between the wavelength plate and the PBS due to the surface tension or the like of an adhesive for bonding the wavelength plate and the PBS to each other. Further, there is also a possibility that an adhesive may enter another optical path, thereby causing an optical loss.